Light-controlling means



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. The present invention relat to means for" controlling the intensity ofa beam of light in accordance with electrically produced changes in amagnetic field. f

A known method of controlling the intensity of a beam of light consistsin passing a beam of plane polarized light through a suitabletransparent medium situated in a mag-- netic field. The light is thenpassed through' n man analyzing device. Then the strength of themagnetic field is changed, .for example by passing. a current through asuitably arranged coil, a rotation ofthe plane of polarization of thelight passing through the field of the light coming from the anal zingevice is therefore changed also. ntensity controlling v means of thiskind are called Faraday cells.

An objection to the use of means 0f th is zu tvpe where it is requiredto transform rapid c ianges in magnetic field into corresponding changesin light:A intensity, as for example when dealin with oscillations atacoustic frequencies, is that the response decreases as z5 thc frequencyincreases, due to the fact that the impedance of the deviceis largely reactive. In order to overcome this diliiculty it has been pro sed toutilize. high frequency curso rents mogiilatcd with acoustic frequencycurrents and to feed these modulated high frequenc currents to anascillatory circuit tuned to t ie frequency of the high frequencyoscillations and containing resistance. The coil of 35 the Faraday cellhas been used as one element 'of the oscillatory circuit. The presentinvention is not concerned with the use of modulated high frequencycurrents but provides means whereb the response at acoustic ,fre-

. 40 quenc'ies. may e made substantially-uniform over a wide range ofsuch frequencies. I According to the present invention there, isprovided a device for controlling the. intcnsity of a beam of polarizedlight 1n ac- 5 cordance with acoustic freque electrical variations inwhich said electrica variations ,are passed through a coil and' therebyprov duce corresponding variations in a magnetic tield through whichsaid beam of light is out undul alecting the responsecharacter- 5passed, wherein electrical correcting means y 100 r rf amplifier havinga micro hone M in its input v circuit onnectedby eads F and E to aliant/Eid;.eelhthrough a. resistance R1, shnnted by a condenser C. TheFaraday cell comprises a coil wound upon a-tube T of insuating materialand has windows at each end. u;`

The tube 'l is filled with-a substance which exhibits the Faraday e'ect,such .for examgle as nitrobenzene. Light from a source polarized by aNicol prism Ng, is passed through 'rho :obo T, through o oooond Niooi nprism N2 and a slit device D on to a travelling film B. Tlie intensityof the li ht falling upon the film B will vary a'cco 'ng to theinstantaneous value of the current in the coil of the Faraday cell.' oThe winding of he cell will be assumed to have an inductance L and aresistance R, which includes any resistance external to the winding buteectively in series therewith excluding Rx. The device will then have asubstantially tlat' topped characteristic, for

a frequency band extending from zero to l. value of the order of that atwhichl a circuit consisting only of an induotance L and a capacitv Cwould resonate, if the value of on R." liea'beftween 0.4%J and 0.5 CWhen Rf lies between these values the series rcsistance R, may be madequite low' so that a minimum ofy power is required to o rate 0lthedevice. For higher values of. the value (5f-fR,I maybe reduced'to0.32:?J with.

comprising resistance and adapted to inf- Another method of carrying-theinvention into effect .involves the us'e of circuits embodying negativeresistances and analogous to the networksvknown es Boucherot network.

In the Journal of the Institution of Elec known or suitable device whichexhibits negl:si

ative resistance characteristics over the de siredl range of current maybe used, such for example as certain known forinsand arv rangements ofthermionic valves and tubes .adapted for maintaining electricaldischarges in gases at low pressure.

A simple arrangement embodying the above principles is shown in Fig. 2.A resistance. R is connected in series with a n'egative resistance Q to'a source of electrical variations which ma comprise the micro. phone Mand a suita le amplifier as above described. The device Q has theproperty that, over. the range of potential differences to which it isto be subjected, its characteristie curve of current against appliedpotential difference slopes in the reverse direction.

to that of a'resistance and that, over this range, the slope of thecurve is substantially constant and equal to Asan example of a suitablenegative resistance device, a four electrode va ve having 'appropriatepo- .tental dierences maintained between Iits electrodes may bementioned. The winding of the Faraday cell is connected across eitherthe'positive or the negative resistance and the currents in this windingwill accordingly be independent of the variations in its 'V impedancewith changes of frequency. In

. winding of the the arrangement shown, the leads E` F arc connected tothe ends of resistance R.

Since it is diiicult in vractice to realize necative resistances' capale of operating satisfactorily at high current values, an amliiier, suchas a thermionic valve amplier, is preferabl included in front pf thearaday cell. In this case the input of the amplier is connected acrosseither. the positive or the negative resistance and the Faraday cellwinding is connected acro the output of the amplifier. 1n this case itis advanta us to arrang ysuitable proportioning o inductances anresstancea. that vthe impedance looking into the ampliiier at the inputend shall be proportional at all frequencies to the'impedan new" isconnected series between the source of electrical variations and theinput termiof the output circui consisting of the :um of the impedanceloo g back into the amplifier and forward into the Faraday cell.

An arrangement having these featur; is shown in Fig. 3. The n tiveristance Q nals of the amplifier arrangement Aq. Theresistance I R. isconnected in shunt with the input terminals of the amplifier arrangementA, and has a value equal and op te to Q. Shunted across the grid circuitof the valve V is a combination ofimpedances comprising a' resistance R,in series with an inductance L,. The resistance R, and the inductance L,have such values that their combined im ancebcars a constant ratio tothe im ance of the output circuit of the amplifier` A, over the range.of frequencies at whichthe device isto operate.V The am- 85 litier A,may comprise any desired numr of .valves of'whichonly ,the lirst valveV, is shown. The leads `E, F s re intended, as in the other two forconnection to s Faraday cell. l v

1. A device for controlling the intensity of a beam of polarized lightin accordance, with a current of varying frequen coinprising alight-controlling-valv sai Vvalve 05 including a coil ada ted to p nce amagnetic field to contro the plane o'f polarization of the light beam,means associated with said coil for supplyin a current of varyingfrequency thereto, an electrical correcting im means includingresistance in circuit with said coil for increasing the relativeresponse at higher frequenciesand maintaining a current through saidcoil that is independent of variations of the impedance of said coil.165

2. A device for controlling the intensity of a beam of polarized lightin accordance with a current of varying frequency com- Y prising a lightcontrollin valve, sai valve including a coil adapted lo produce a' mag-11. netic field 4to control the plane o f polarization of the lightbeam, meansfor supplying a. current ofvarying frequency .to said coil,and electrical correcti v means comprising 'tive and negativeresistances Vforincreas- 115 ing the relative response of said coil athigher frequencies to compensate for variations of the impedance of saidcoil.

3. A device for controlling the intensity K01E a beam of polarized lightin accordance 120 tion of the light beam, means for supplyin 125 'acurrent of varying frequency torsaid roi and electrical correcting meanscomprising 'tive and negativo resistances for increasina' the relativeresponse of said coil at higher freq ueaces, one of said resistance! lllbeing connected effectively in. series with Said coil and the othereffectively in shunt with said coil.

` 4. A device for controlling the intensity of 'a beam of polarizedlight in accordance .w'itha currentof` varying frequency compri-sing athermonic valve amplifier adapted to produce an 'output current ofvarying frequency, a light controlling valve including a coil connectedin the'output circuit of said amplifier and. adapted to produce` a.

magnetic 'field to control the lane of polarization ofthe light beam,and) electrical correcting means comprising resistance for maintaining acurrent through said coil that is independent of variations of impedanceof the coil.

. 5.V A device for controlling the intensity of a beamlof polarizedlight in accordance with a current of varying frequency comprising athermionic valve amplifier adapted to produce an output current ofvarying fre,

quency, a light controlling valve includinfr a' coil connected to the.output circuit osaid amplifier and adapted to produce a magnetic fieldto control'the plane -of polarization of the light beam, and electricalcorrecting means comprising positive and nega tive resistances forincreasing the relative response of said-coil at higher frequencies jtocompensate for variations of im edance.

6. vA device for controlling the intensity lof 'a beam of polarizedlight in accord-ance with a current of varying frequency comprising athermionic valve amplifier adapted dance of the outputcincuit of'saidampli-l iir in. the frequency range.

8. A .device -for controlling the intensity l of a. of polarized lightin accordance with a current of varying frequency comprisd 7d ing ath'ermionic valve amplifier adap'ted'to produce an output current ofvarying frequency, a light controlling valve includin a coil connectedin the output circuit of sai amplifier and adapted to produce a.magnetic 75 field to control the plane of polarization of the lightbeams, electricalcorrecting means i to increase the relative of saidcoil at higher frequencics,said correcting means comprising positive andnegative resistances one effechvely in series with said coil and oneeffectively in shunt with-said,coil and impedance means in shunt 'withthe input 1" circuit of said amplifier, said impedance havingasubstantial-ly constant ratio to the ima5 pedante of the output.circuit of the amplitier over the'frequency range.

In testimony whereof, we have signed our names to this specication atLondon England, this 8th day of January, 1929, an 0o I at Camden, NewJersey, this 25th day of January, 1929. v

to produce an output current of varying fre- A quency, a lightcontrolling valve including a coil connected in the' output circuit ofsaid A amplifier and adapted to produce a magnetic field to control thepl ane of polarization ofthe i light beam, and electrical correctinmeans. comprising positive and negative 'resistances to increase therelative response of said coil at higher frequencies, one of saidresistances beingvei'e'ctive-ly in series with said coll and the otherbeing electively in shunt 'with said coil.

7. A device `for controlling the intensity l of a beam of polarizedlight. in accordance with a current of varyingfrequency'gcompri'sing athermiouic valve amplifier means 65 ing a 'substantially constant ratioto the im-

